1. Field of the Invention
The present invention relates to a measuring chip that is employed in a surface plasmon resonance measurement apparatus for quantitatively analyzing the properties of a substance in a liquid sample by utilizing surface plasmon excitation.
2. Description of the Related Art
If free electrons vibrate collectively in a metal, a compression wave called a plasma wave will be generated. The compression wave, generated in the metal surface and quantized, is called a surface plasmon.
There are various kinds of surface plasmon resonance measurement apparatuses for quantitatively analyzing a substance in a liquid sample by taking advantage of a phenomenon that the surface plasmon is excited by a light wave. Among such apparatuses, one employing the “Kretschmann configuration” is particularly well known (e.g., see Japanese Unexamined Patent Publication No. 6(1994)-167443).
The surface plasmon resonance measurement apparatus employing the aforementioned “Kretschmann configuration” includes (1) a dielectric block formed into the shape of a prism; (2) a metal film, formed on a surface of the dielectric block, for placing a sample thereon; (3) a light source for emitting a light beam; (4) an optical system for making the light beam enter the dielectric block so that a condition for total internal reflection is satisfied at the interface between the dielectric block and the metal film and that various angles of incidence, including a surface plasmon resonance condition, are obtained; and (5) photodetection means for measuring the intensity of the light beam totally reflected from the interface to detect surface plasmon resonance (SPR).
In order to obtain various angles of incidence in the aforementioned manner, a relatively thin light beam may be deflected to strike the above-mentioned interface, or relatively thick convergent or divergent rays may strike the interface so that they have components which are incident at various angles. In the former, a light beam whose reflection angle varies with the deflection thereof can be detected by a small photodetector that is moved in synchronization with the deflection, or by an area sensor extending in the direction where the angle of reflection varies. In the latter, on the other hand, rays reflected at various angles can be detected by an area sensor extending in a direction where all the reflected rays can be received.
In the above-described surface plasmon resonance measurement apparatus, if a light beam strikes the thin film layer at a specific incidence angle θsp greater than a critical incidence angle at which total internal reflection (TIR) takes place, an evanescent wave having electric field distribution is generated in a liquid sample in contact with the thin film layer. The evanescent wave excites the above-described surface plasmon at the interface between the thin film layer and the liquid sample. When the wave vector of the evanescent wave is equal to the wave number of the surface plasmon and therefore the wave numbers between the two are matched, the evanescent wave resonates with the surface plasmon and the light energy is transferred to the surface plasmon, whereby the intensity of the light totally reflected from the interface between the dielectric block and the metal film drops sharply. This sharp intensity drop is generally detected as a dark line by the above-described photodetection means.
Note that the aforementioned resonance occurs only when an incident light beam is a p-polarized light beam. Therefore, in order to make the resonance occur, it is necessary to make settings in advance so that a light beam can strike the aforementioned interface as a p-polarized light beam.
If the wave number of the surface plasmon is found from a specific incidence angle θsp at which attenuated total reflection (hereinafter referred to as ATR) takes place, the dielectric constant of a sample to be analyzed can be calculated by the following Equation:                                           K            sp                    ⁡                      (            ω            )                          =                              ω            c                    ⁢                                                                                          ɛ                    m                                    ⁡                                      (                    ω                    )                                                  ⁢                                  ɛ                  s                                                                                                  ɛ                    m                                    ⁡                                      (                    ω                    )                                                  +                                  ɛ                  s                                                                                        (        1        )            where Ksp represents the wave number of the surface plasmon, ω represents the angular frequency of the surface plasmon, c represents the speed of light in vacuum, and εm and εs represent the dielectric constants of the metal and the sample, respectively.
If the dielectric constant εs of the sample is found, the concentration of a specific substance in the sample is found based on a predetermined calibration curve, etc. As a result, the specific substance can be quantitatively analyzed by finding the specific incidence angle θsp at which the intensity of reflected light drops sharply.
In the conventional surface plasmon resonance measurement apparatus employing the aforementioned system, the metal film on which a sample is placed must be exchanged each time a measurement is made. Because of this, the metal film is fixed on a first dielectric block in the form of a plate, and a second dielectric block in the form of a prism is provided as an optical coupler for making the aforementioned total internal reflection occur. The first dielectric block is integrated with a surface of the second dielectric block. The second dielectric block is fixed with respect to an optical system. The first dielectric block and the metal film are used as a measuring chip. In this manner, the measuring chip can be exchanged every time a measurement is made.
In addition, a leaky mode measurement apparatus is known as a similar measurement apparatus making use of ATR (for example, see “Spectral Research,” Vol. 47, No.1 (1998), pp. 21 to 23 and pp. 26 to 27). This leaky mode measurement apparatus includes (1) a dielectric block formed into the shape of a prism; (2) a cladding layer formed on a surface of the dielectric block; (3) an optical waveguide layer, formed on the cladding layer, for placing a sample thereon; (4) a light source for emitting a light beam; (5) an optical system for making the light beam enter the dielectric block at various angles of incidence so that a condition for total internal reflection is satisfied at the interface between the dielectric block and the cladding layer; and (6) photodetection means for measuring the intensity of the light beam totally reflected from the interface to detect the excited state of a waveguide mode, i.e., the state of ATR.
In the above-described leaky mode measurement apparatus, if a light beam strikes the cladding layer through the dielectric block at incidence angles greater than a critical incidence angle at which total internal reflection (TIR) takes place, the light beam is transmitted through the cladding layer. Thereafter, in the optical waveguide layer formed on the cladding layer, only light with a specific wave number, incident at a specific incidence angle propagates in a waveguide mode. If the waveguide mode is excited in this manner, most of the incident light is confined within the optical waveguide layer, and consequently, ATR occurs in which the intensity of light totally reflected from the aforementioned interface drops sharply. The wave number of the light propagating through the optical waveguide layer depends upon the refractive index of the sample on the optical waveguide layer. Therefore, the refractive index of the liquid sample and the properties of the liquid sample related to the refractive index can be analyzed by finding the above-described specific incidence angle θsp at which ATR takes place.
In the leaky mode measurement apparatus, as with the aforementioned surface plasmon resonance measurement apparatus, a first dielectric block is fixed with respect to the optical system, and the cladding layer and the optical waveguide layer are formed on a second dielectric block and used as a measuring chip. When a sample is exchanged, only the measuring chip can be exchanged.
However, in the case where the conventional measuring chip which is exchangeable is employed, a gap occurs between the first dielectric block and the second dielectric block and the refractive index becomes discontinuous. To prevent the discontinuity, it is necessary that the two dielectric blocks be united through an index-matching solution. The operation of uniting the two dielectric blocks into an integral body is fairly difficult, and consequently, the conventional measuring chip is not easy to handle in making a measurement. There are cases where measurement is automated by automatically loading a plurality of measuring chips into a turret, then rotating the turret, and automatically supplying the measuring chips to a measuring position where a light beam enters the measuring chip. In such a case, the loading and removal of the measuring chips are time-consuming. As a result, the efficiency of the automatic measurement is reduced.
In addition, there is a possibility that the conventional measuring chip will have a detrimental influence on the environment, because it uses an index-matching solution.
In view of the circumstances mentioned above, there has been proposed a surface plasmon resonance measuring chip that can be easily exchanged without using an index-matching solution (see U.S. Patent Laid-Open No. 20010040680). The measuring chip comprises (1) a dielectric block; (2) a thin film layer, formed on a surface of the dielectric block, for placing a sample thereon; (3) a light source for emitting a light beam; (4) an optical system for making the light beam enter the dielectric block so that a condition for total internal reflection is satisfied at an interface between the dielectric block and the thin film layer and that the light beam has incident components at various angles; and (5) photodetection means for detecting the intensity of the light beam totally reflected from the interface to detect the state of ATR. The dielectric block is formed as a single block, which includes an entrance surface through which the light beam enters the dielectric block, an exit surface through which the light beam emerges from the dielectric block, and a surface on which the thin film layer is formed. The thin film layer is integrated with the dielectric block.
Note that in the case where the measuring chip is used in the above-described surface plasmon resonance measurement apparatus, the above-described thin film layer is constructed of a metal film. In the case where it is used in a leaky mode measurement apparatus, the thin film layer is constructed of a cladding layer and an optical waveguide layer. In addition, the dielectric block constituting the measuring chip preferably has a sample holder for holding a sample on the thin metal film, formed by surrounding the space above the thin metal film from the sides thereof.
The above-described dielectric block, incidentally, is generally injection-molded into the shape of a truncated quadrangular pyramid, a square pole, etc., and is suitably employed. However, in a conventional measuring chip equipped with such a resin dielectric block, there are cases where the reproducibility of measured data is not good.